This invention relates to an improved resin composition and more particularly to an improved styrene/butadiene resin composition useful for making electrostatographic toner compositions.
Styrene/butadiene resins are known in the art. For example, U.S. Pat. Nos. 5,166,028 and 4,469,770 describe styrene butadiene copolymers, methods for their manufacture, and their use in toners and developers.
Styrene/butadiene resins may be produced by a variety of different methods. For example, U.S. Pat. No. 5,166,028 discloses a dispersion polymerization process for producing such resins. The resin is prepared by dispersion polymerizing a homogeneous organic or aqueous/organic mixture comprising one or more solvents, one or more stearic stabilizers such as poly(vinylpyrrolidone), one or more initiators, styrene monomer, butadiene monomer and a surfactant. However, the known dispersion polyermization processes have several disadvantages in the production of styrene/butadiene toner resins. For example, the dispersion polymerization process requires the use of one or more solvents, which must be removed from the reaction medium and the product particles after polymerization has completed. However, not all of the solvent may be removed from the particles, and even the small amounts of solvent that remain can affect the resin particle properties. In addition to solvent, the resin particles also include amounts of the stearic stabilizer, such as poly(vinylpyrrolidone), which may also affect the particle properties. Additionally, it is more difficult in the dispersion polymerization process to control the molecular weight distribution.
U.S. Pat. No. 4,469,770 also describes an emulsion polymerization process for producing styrene/butadiene resins. Emulsion polymerization is a well-known industrial process. Emulsion polymerization generally comprises forming an emulsion of a surfactant and the monomer in water, then polymerizing the monomer in the presence of a water soluble initiator. Generally, however, the emulsion polymerization process is used to make very high molecular weight polymers (e.g., 100,000 to several million) with a low molecular weight distribution (e.g., 2 to 4). The molecular weight is very high because the particles are so small that they encounter initiator very rarely. Furthermore, the high interior viscosity promotes a gel effect, whereby the growing chains have difficulty terminating because the radicals cannot move toward each other before growing to great length. Chain transfer agents can be used to decrease the molecular weight, but added chain transfer agent also greatly increases the molecular weight distribution.
Experimentation in the art of toner resin production has focused upon narrow molecular weight regions with narrow molecular weight distributions, as those two variables greatly affect the gloss and fix properties of a toner resin. In many cases a trade-off is necessary between the gloss and fix properties of the toner, and such a trade-off usually results in an increase in the fusing temperature. For example, a toner designed with high gloss properties often has poor fix behavior. Either the toner is too brittle and has poor mechanical properties or the particles are not well coalesced to each other and thus do not adhere well to the paper. In other cases, the fix properties of the toner may be very good, but only at the expense of the gloss properties. Efforts to improve both the gloss and the fix properties of toner resins have been undertaken, but have met minimal success.
A toner exists that utilizes a styrene/butadiene latex available as Pliotone from the Goodyear Tire and Rubber Company. This resin has a number average molecular weight (Mn) of about 13,900, a weight average molecular weight (Mw) of about 97,500, a molecular weight distribution (MWD) of about 7 and a glass transition temperature (Tg) of about 60.3.degree. C. However, the resin has a problem with gloss properties and projection efficiency, and requires a higher fusing temperature to produce acceptable toner results.
A significant improvement in gloss and fix properties should be obtainable by modifying the toner resin composition. For example, a fraction of projection is lost due to the rough surface of the fused toner image. This rough surface is believed to be due to the poor melt properties of the toner.